Possibilities of detecting a position of resonantly driven or moved MEMS devices (MEMS=micro-electromechanical system), such as of resonantly driven micromirrors, are limited. Basically, one can detect either the movement of an actuator driving the micromirror or the movement of the micromirror directly. The most common sensor principles for position measurement of a micromirror are based on piezoresistive or metallic strain measuring strips (strain gages) or on capacitive or optical methods.
Frequently, piezoresistive strain measuring strips, metallic strain measuring strips and capacitive measurement principles only detect the actuator position rather than the actual position of the micromirror, while optical methods measure the real position of the mirror. However, the above mentioned measurement methods have their specific limitations, as will be stated below.
Integrating piezoresistive sensors may use specific production materials such as doped polysilicon, for example, in order to generate a change in resistance by the mechanical tension present in the device. This results in limited implementability [C. Zhang, G. Zhang, and Z. You, “A two-dimensional micro scanner integrated with a piezoelectric actuator and piezoresistors,” Sensors, vol. 9, no. 1, pp. 631-644, 2009]. In addition, piezoresistors made of a semiconductor material are sensitive also to light incidence and changes in temperature, which strongly limits the use of this class of sensors in a microdevice.
Metallic strain measuring strips benefit from the fact that many metals have a piezoresistive behavior such that their respective electric resistance changes upon deformation. Metallic strain measuring structures are significantly easier to implement than piezoresistors made of semiconductive material while having significantly poorer sensitivities. Thus, their usage involves a sufficiently large strain of either the actuators or the spring structures between actuator and micromirror [M. Cueff, E. Defaÿ, G. Le Rhun, P. Rey, F. Perruchot, A. Suhm, and M. Aïd, “Integrated metallic gauge in a piezoelectric cantilever,” Sens. Actuators Phys., vol. 172, no. 1, pp. 148-153, December 2011].
Eventually, the amount of the change in resistance depends decisively on the design of the MEMS device. However, many MEMS designs lack suitable structures which on the one hand have large deformations and on the other hand provide sufficient space for a strain measuring strip [U. Baran, D. Brown, S. Holmstrom, D. Balma, W. O. Davis, P. Muralt, and H. Urey, “Resonant PZT MEMS Scanner for High-Resolution Displays,” J. Microelectromechanical Syst., vol. 21, no. 6, pp. 1303-1310, December 2012]. In such MEMS devices, the usage of strain measuring strips is therefore associated with poor sensitivity and poor resolution.
Electrostatic drives use mutually attractive areas of different voltage levels as actuators. Conversely, these electrode areas can also be used as components of capacitive sensors, the sensor signals of which in turn provide information about the distances of the electrodes, which eventually provides a statement regarding the position of the micromirror [H. G. Xu, T. Ono, and M. Esashi, “Precise motion control of a nanopositioning PZT microstage using integrated capacitive displacement sensors,” J. Micromechanics Microengineering, vol. 16, no. 12, p. 2747, December 2006], [T. von Wantoch, C. Mallas, U. Hofmann, J. Janes, B. Wagner, and W. Benecke, “Analysis of capacitive sensing for 2D-MEMS scanner laser projection,” in Proc. SPIE 8977, MOEMS and Miniaturized Systems XIII, 2014, p. 897707]. Here, either the position of the actuators or the position of the micromirrors can be detected directly. However, what is disadvantageous here are the comparatively low signal strengths and the interference of the signals by crosstalk. Not least, the comparatively large electrode areas and their small distances cause significantly increased aerodynamic friction of the quickly moving micromirror, which is accompanied by a significant decrease in the maximum deflection of the resonant micromirror.
Optical measurements present a possibility of directly detecting the movement of the mirror plate. Here, however, an external light source, beam processing, and a detector (laser diode) may be used, which impair the compactness of a device [S. Richter, M. Stutz, A. Gratzke, Y. Schleitzer, G. Krampert, F. Hoeller, U. Wolf, L. Riedel, and D. Doering, “Position sensing and tracking with quasistatic MEMS mirrors,” 2013, p. 86160D], which eventually results in complex and expensive constructions.
In addition, micromirrors as well as methods for producing same are known [S. Gu-Stoppel, J. Janes, H. J. Quenzer, U. Hofmann and W. Benecke, “Two-dimensional scanning using two single-axis low voltage PZT resonant micromirrors”, Proc. SPIE 8977, MOEMS and Miniaturized Systems XIII, 897706 (7 Mar. 2014)].